High-fidelity sound reproducing apparatus and method of improving the fidelity thereof



March 17, 1936.

H. A. WHEELER ET AL 2,034,014 HIGH FIDELITY SOUND REPRODUCING APPARATUSAND METHOD OF IMPROVING THE FIDELITY THEREOF Filed April 4, 1955 3Sheets-Sheet 1 III INVENTORS. HAROLD A. WHEELER VERNON E. WHITMAN 1 VATTORNEY March 17, 1936. A WHEELER ET AL 2,034,014

HIGH FIDELITY SOUND REPRODUCING APPARATUS AND METHOD OF IMPROVING THEFIDELITY THEREOF Filed April 4, 1935 5 Sheets-Sheet 2 INVENTORS HAROLDA. WHEELER VERNON f. W/l/T/VA/V NELSON P. (1455 ATTORNEY.

March 17, 1936. A, WHEELER E L 2,034,314

HIGH FIDELITY SOUND REPRODUCING APPARATUS AND METHOD OF IMPROVING THEFIDELITY THEREOF Filed April 4, 1955 3 Sheets-Sheet 5 SOUND RE PRODUCING05 VIC E frequency range.

Patented Mar. 17, 1936 PATENT OFFICE HIGH-FIDELITY SOUND REPRODUCING AP-PARATUS AND METHOD THE FIDELITY THEREOF Harold A. Wheeler, Great Neck,Vernon E. Whitman, Flushing, and Nelson P. Case and William 0. Swlnyard,Bayside, N. Y., assignors to Hazeltlne Corporation, a corporation ofDelaware Application April 4,1935, Serial No. 14,600

17 Claims. Our invention relates to sound-reproducing ,apparatus and,whileit is of general application,

it is particularly suitable for use in connection with radio broadcastreceivers.

In the satisfactory reproduction of sound, as from a radio broadcastreceiver, two distinct problems arise. One is that of the fidelity ofreproduction, that is, the uniform and distortionless reproduction ofsound throughout the audible Among the important factors present to agreater or lesser'extent in the soundreproducing devices of the priorart tending to impairthe fidelity of sound reproduction, might bementioned the non-uniform frequency-re-' sponse, characteristic of theprimary sound-reproducing element; the resonant or non-uniform frequencycharacteristic of the housing or cabrnet in which such adevice isusually mounted; and interference patterns set up by waves'gerrerated bydifferent portions of the sound-reproducing device.

A second important problem is that of the ef-- fectiveness of the deviceand the closely related problem of the distribution of the sound wavesin their projection from the device to the audience,

usually involving the distribution over a room or auditorium. Among theimportant factors dis advantageously affecting any arrangements of theprior art in this respect may be mentioned the lack of proper directionof the sound waves from the device into the room; *the reflectionofthesound waves from the walls, floor and ceiling,

giving rise to interference patterns; and the nonuniform angular, andimproper vertical distribution of the sound waves as they areprojectedinto 'the room, resulting in dead spots, that is, areas of verylow sound intensity, accompanied by other areas of abnormally highintensity.

' stantlally reduced and which will reproduce device having anapproximately uniform fre- It is an object of our invention to provide asound-reproducing device in which the short-= comings of thearrangements of the prior art, discussed above, are largely eliminatedor subsound waves with a maximum fidelity an'd effectiveness.

More specifically, it is an object of ourinvention to provide ahigh-fidelity sound-reproducing sated for externally to thesound-reproducing device, per se, preferably in the electrical networkto which the device is connected.

or the casing cavity. v In order still further to improve the fidelityof vided a high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, such as a corner of a room. Within the casing ismounted a large-diaphragm type of sound-reproducing device disposed todischarge, preferably vertically, into the sound cavity at a pointdisplaced from the vertex. sound cavity includes means for directing ordefleeting the sound waves generated by the device outwardly from thevertex with a substantially uniform angular distribution and a propervertical distribution, as determined by the physical relationship of thesound-reproducing device to an audience.

More specifically, in accordance with our invention there-is provided ahigh-fidelity soundsubstantially to the corner of a room so that thewalls and floor or ceiling constitute effectively a continuation of thewalls of the sound cavity. A partition separates the cavity from therest of'the casing and a large-diaphragm type sound-reproducing deviceis disposed in an opening in the partition to discharge vertically intothe cavity. A conical deflector is positioned between thesound-reproducing device and the opening of the sound cavity and iseffective to direct outwardly from the vertex. the sound waves generatedby the device. One or more additional segmental coni- Thev caldeflectors and a plurality of vertical fins radi- "ating from a point ofthe interior of the cavity substantially on the axis of thesound-reproducin'g device effect a uniform angular distribution and aproper vertical distribution of the sound waves generated by the deviceinto the room-0r auditorium in which the apparatus is placed. Theremaining portion of the casing comprises a rigid, substantiallyanti-resonant structure lined with a sound-absorbent material. A largemass of sound-absorbent material is disposedln the end of the casingopposite the sound cavity in the form of a pyramid having as its basethe end of the casing and having its vertex in the corner including thevertex of the sound reproduction ofapparatus embodying our invention theelectrical transmissionnetwork supplying energy to the sound-reproducingdevice is compensated for resultant or residual lack of unir formity inthe frequency-response characteristic of the apparatus by associatingtherewith. an

auxiliary network. Such a 'network may comprise oneor more seriesresonant circuits connected in parallel and having natural frequenciesorresponding tothe peaks of the frequency-respouse characteristic of thedevice and one or more parallel resonant circuits having naturalfrequencies corresponding to the dips of the frequency-responsecharacteristic.

For a better understanding of our invention,

together withother'and further objectsthereof,

reference is had to the following description, taken in conection withthe accompanying drawings, and its scope will be pointed out in theappended claims.-

Referring now to the drawings, Fig. 1 is a view in perspective of "asound-reproducing apparatus" constructed in accordance withourinvention; Fig. 2 is a longitudinal section of the apparatus ofFig. 1taken along the lines 2-.-2 of 'Fig. 4; Figs, 3 and 4 arecross-sectionalviews of the ap paratus of Fig. 1 taken along thelines3-8 and. 4-4, respectively, of Fig. 2; Fig; Sirepresents certainoperating characteristics of the apparatus of Figs. 1-4 inclusive,- andcertain corrections applied thereto in accordance with our invention;while Fig. 6 is a'schematic circuit diagram of a complete radiobroadcast receiver, to which the sopndereproducing-apparatus of ourinven tion may suitably be-applied, including a network for compensatingfor any residual lack of uniformity in the frequency-responsecharacteristic of our improved sound-reproducing apparatus.

Referring now more particularly to Figs. 1-4,

inclusive,- of the "drawings, there .is shown a sound-reproducingapparatus embodying our. inv vention comprising a casingor housing lterminating at its lower end in a'sound cavity I I which is separatedfrom the upper portion. of the easing by a partition I2. An opening Ilais pro-' vided in the partition in which or over which is mounted asound-reproducing device or element I 3,0! the large-diaphragm type. Bythe term large-diaphragm type sound-reproducing de me is meant a devicehaving a relatively large vibrating diaphragm, usually conical, which,because of its size, produces directly sound waves of sumclent intensityfor usual purposes. Such a device is to-be distinguished from thesmall-dia- 1 phragm type of sound-reproducing device which is usefulonly in connection with a sound-amplifying device, such as a horn. Thesound cavity II is'open on two sides while its two rearsides terminatein the comer Il a, shaped to conform corner of the casing I0, asindicated at, Illa.

to the vertex of a trihedral angle,-such asthe corner of;a room, theintersections of the walls of such a 'room being-indicated schematicallybythe lines I4 of Fig. 1. with such' an arrangement the walls and flooror ceiling constitute effectively an extension of the sound cavity -IIand secure the maximum effectiveness .of the sound-reproducing deviceI3. I

In order properly to direct and uniformly to distribute sound waves'generatedby the device I3, there is disposed in the sound cavity. II aconical deflector I the axis of which lies in the vertical through thevertex of the sound cavity II and the base of which is'substantially'coextensive with the base of the sound cavity II. To

this end, and also for purposes described hereinafter,- it may bedesirable to-cut of! the front angle of the conical deflector I 5 is notcritical but is determined, among other considerations, by the height ofthe partition I:

above the base of the should be such as todirect the sound waves gen--sound waves.

cavity I I and, preferably,

erated by the device I3v substantially to the head of a listener locatedin the center of the room in which the apparatus is to be utilized. Thisvertical angle is preferably of the order of 130, although it maybevaried within limits of about 120 from this value. The deflector I5serves principally to reflectthe downwardly projected sound wavesoutwardly into the room.

prove, the angular distribution of the sound waves, there are alsoprovided a plurality of vertical fins, in this instance the two flns I1and I8,

extending between the conical deflector I5 and the partition I2 andradiatingoutwardly from r substantially the center of the sound cavity,and/or center of the sound-reproducing device IS.

The fins I1 and I8 may terminate in a fin 19 extending to the rear comerof the sound cavity I I.

The conical deflectors I5 and I6 and the verticalflns I I, I 8 and I9must be of suflicient rigidity to prevent substantial mechanicalvibration in the audio-frequency range, and to'thise'nd they maycomprise sheets of moderately heavy gauge metal, welded, soldered, orotherwise fabricated into a unitary structure.

The upper portion of the casing lfl above the partition I2 constitutesav closed sound-absorb ing chamber to prevent'interference'of the soundwaves generated at the rear of the diaphragm of the device I3 with themain sound waves which it is desired to utilize. This portion of thecasing is also preferablyconstructed of relatively heavy.

rigid, anti-resonant walls and may include brac- I ing structure toassist in securing this characteristic. This portion of the casing issubstantially completely lined with a sound-absorbing sheet material 20which may be of any suitable type, as, for example, a fibroussheetmaterial commercially available as Acousti-Celotex.' In addition,

a screen or other sonnd-pervious sheet-or strip 2I divides off from theupper portion of the 09.5-- ing a pyramidal cavity the baseof whichconstitutes the top of theca'sing III and the vertex of which lies atthe rear corner of the casing; that is, in a vertical through the vertexof the sound cavity I I.. This chamber 22 is filled with a sound-'absorbent material such as rock wool or slag wool,

and, if desired, a fabric covering may be provided for the screen 2| toretain dust, dlrt and small particles of the sound-absorbent material.

It is believed that the operation of our improved sound-reproducingapparatus will be clear from the foregoing description. In brief, thesound waves generated by the device I3 and projected downwardly into thesound cavity II are deflected or reflected from the conical deflector I5outwardly into the room. The conical configuration of the deflector I5tends to produce substantially uniform angular distribution of the Asstated above, the inclination of the deflector I5 should preferably besuchas to deflect the sound waves upwardly at such anangle as to reachthe head of a listener in substantially the center of a-room in whichthe apparatus is to be utilized; This result maybeseto provide a moreequal or uniform angular distribution of the sound waves, the deflectorI5 being effective also to increase the uniformity of the verticaldistribution of the sound waves. as they leave the sound cavity I I. Ithas been found that the fins l1 and I8 and deflector l5 are particularlyeffective over a range of audio frequency of from 600 to l200cycles.

in a particular case.

The portion of the casing 10 above the sound cavity I! is effective toabsorb sound waves generated by the rear of the diaphragm of the devicel3, and this absorption should be effected without mechanical vibrationof any of the casing structure. Such mechanical vibration is substantially avoided, as described above, by utilizing relatively rigidwalls, with or without bracing, and by lining the walls with asound-absorbent material. The resonant properties of the casing avoidedby this means should be distinguished; however, from the resonantproperties of the enclosed body of air. Resonance of the body of air isminimized by the pyramidal mass of sound-absorbent material 22, whichrenders the portion of the casing above the baiiie I! of non-uniformdepth throughout its cross-sectional area, and by the beveled portionliia of the easing. This beveling of the front corner of the casingsubstantially eliminates any parallel surfaces in this portion of thecasing and thereby substantially eliminates the resonance effects orstanding waves; facilitates high-angle reflection and aids in the'betterutilization of the corner space of the room.

'The dimensions, both relative and absolute, of the above-describedapparatus will be determined largely by therequirements of eachparticular type of installation, although the following relationshipsrepresent those found to be satisfactory The included angle between theside walls 01 the sound cavity is, of

course, that determined by the trihedral angle constituting thecorner-of the room in which the apparatus is to be used, and is usually90. The vertical opening of the sound cavlty- H is preferably of theorder of magnitude of the diameter of the diaphragm of the device l3.The total height of the housing It is not critical but is preferablyatleast 5 or 6 times the diameter of the diaphragm and should providesufilcient volume to build up the low notes generated by the device. Thediaphragm of the-device I3 is preferably parallel to the floor, althoughit maybe inclined slightlythereto, the inclination of the. deflectors l5and i6 being correspondingly modified.

While the casing l0, and'particularly the soun cavity H thereof, hasbeen illustrated as conforming substantially to a righttrihedral anglesuch as the corner of a room, it will be apparent 1 'that it may conformto any polyhedral angle, or, in general, to any conical shape, in caseit is to" verted with the sound cavity immediately adjacent the ceilingand the same principles of oper ation will govern the performance of theapparatus.

The frequency-response characteristic of the sound-reproducing apparatusdescribed above,

as determined by a microphone situated in such a way with respect to theapparatus as to simulate normal operating conditions, is shown by curveA of Fig. 5. From this curve it is seen that the fidelity ofreproduction is reasonably uniform over the audible frequency range offrom 70 to 6000 cycles. In this same figure, curve B represents acorrection curve which, if applied to the apparatus whose characteristicis represented by the curve A, would produce absolutely uniformfrequency response. The curve B is, obviously, the inverse of the curveA. Curve C, in this same figure, represents a correction curveobtainable by a frequency-selective network, to

. be described more particularly hereinafter, ap-

plied to the electrical transmission system to which thesound-reproducing device is connected. It is seen that this represents avery close approximation to the ideal correction curve B. The resultantof curves A and C is shown in curve D, which represents thefrequency-response characteristic of a complete system embodying ourinvention. The average of the curve I) has been moved upwardly from'theother curves to avoid confusion and since it is relative, and not abso-'lute, values of the response which are important. It is seen that thecurve D does not depart from its average value more than about i3decibels.

An electrical transmission system by means of which the characteristicsof Fig. 5 may be obtainedis shown in Fig. 6 of the drawings, which is aschematic diagram of a complete radio broadcast receiver modified inaccordance with our invention. This system includes an input circuit 30for connection between an antenna and ground, a radio-frequencyamplifier 31, an oscillator-modulator 32, an intermediate-frequencyamplifier 33, and a detector 34, all coupled in-cascade. The output ofthe detector 34 is fed to an audio-frequency amplifier including thevacuum tubes 35 and 36 connected in any conventional manner, thecircuits of which form 'no part of our present invention. Theaudio-frequency amplifier is, in turn, coupled to a pushpull poweramplifier including vacuum tubes 31 and 38 and associated circuitelements, also connected in a conventional manner, which, in turn, areconnected to sound-reproducing device 39, which is of the typeillustrated in, and described in connection with Figs. 1-4 inclusive.

In order to modify the frequency-response characteristics of the abovesystem to. produce the correction represented by the curve C of'Fig. 5,a frequency-selective network 40, represented by the dotted outline, isincluded in the cathode circuit of the vacuum tube 35. This networkcomprises a parallel resonant circuit consisting of an inductance 4| anda condenser 42, shunted by a resistor 43, connected in series with asecond parallel resonant circuit consisting of an inductance 44 and acondenser 45. Connected in parallel with the two serially connectedparallel resonant circuits just described, through a blocking condenser46, are the pairs of series resonant circuits comprising inductances41,49, 5i and 53 and condensers 48, 50, 52 and 54, respectively.Further, there is connected in parallel with the voice coil or thesound-reproducing device 39 a frequency-selective network 55,represented in dotted outline, comprising series resonant in-. ductance56 and'condenser 51.

It will be understood that the impedance constants of -the severalelements of the above-dc: scribed frequency-selective networks will bedependent entirely upon the particular frequency- I apparatus, per se.

response characteristic of the sound-reproducing v 13y way of exampleonly, as illustrative of a frequency-selective network for producingthe'correction curve C of Fig. 5, the frequency-selective networks mayhave the following values:

Inductance 41 .12 henries Inductance 44 75 henries Inductance 47 1'7henries Inductance 49 5.8 henries Inductance 51 5 henries Inductance 5 31.5 henrles Inductance '56 88 microhenries' Condenser 42 .027microfarads Condenser .15 microfa'rads Condenser 46 .01 microfaradsCondenser 48 .5 microfarads Condenser '50 ".07 microfarads Condenser 52.007 microfarads Condenser 54' .006 microfarads Condenser 57 3microfarads Resistor 43 3000 ohms.

, As indicated in the drawings, the parallel resonant circuit Al, 42produces the dip in curve C at'2800 cycles; the parallel resonantcircuit 4 5, 45 produces the dip at 150 cycles; the series resonantcircuit 56, 51 in the circuit of the soundreproducing device produces adesired dip at 10,000 cycles. 0n the other hand, the series resonantcircuit 41, 48 produces the peakat 55 I cycles; the circuit 49, producesthe peak at 250 cycles; the circuit 5|, 52 produces the peak at 850cycles; the circuit 53, 50 produces the peak at 1700 cycles; and theoutput circuit including the reactor and condenser 51 produces the Thecircuit constants for procuring the correction curve C may be determinedempirically or computed in any of several well known manners. Fromtheforegoing'description it is apparent that, in accordance with ourinvention,

- there is provided a sound-reproducing apparatus having a nearlyuniform frequency-response characteristic, together with a method of,and

apparatus for, compensating for any residual non-uniformity in thefrequency-response characteristic within any desired limits, whereby thedeviation of the frequency-response curve from its average value may bemaintained within any assigned error, by proper design of thefrequencyselective network.

.' tions as fall within the true While we have described what we atpresent consider the preferred embodiment of our invention, it will beobvious to those skilled in the I of a trihedral angle, as a corner of aroom, a large-diaphragm type sound-reproducing device disposedto radiatesound waves into said cavity at a point displacedfrom said vertex, andmeans for directing the sound waves radiated by said 2. A high-fidelitysound-reproducing apparatus comprising a'casing including a sound cavityshaped to conform substantially to the vertex of a trihedral angle, as acornerof a room, a large-diaphragm type sound-reproducing devicedisposed to radiate sound waves approximately vertically into saidcavity, and means for directing outwardly and distributing uniformly thesound waves radiated by said device.

- 3. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a largediaphragm typesound-reproducing device disposed to radiate sound wavesapproximatelyvertically into said cavity, and aconical deflector positioned belowsaid device and effective to direct and distribute sound waves radiatedby said device outwardly from said vertex.

4. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, asa corner of a. room, a largediaphragm typesound-reproducingdevice disposed to radiate sound waves approximatelyvertically into said cavity, and rigid conical deflecting meanspositioned below said device and efiective to direct and distributesound waves radiated by said device outwardly from said vertex.

5. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a largediaphragm typesound-reproducing device disposed to radiate sound waves approximatelyvertically into said cavity, and a rigid conical deflector having itsaxis lying in a vertical intersecting said vertex and substantiallycoextensive with said cavity, said deflector being efiective to directand distribute outwardly from said vertex the sound waves radiated bysaid device.

6. A high-fidelity sound-reproducing apparatus comprising a casingincluding a'sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a largediaphragm typesound-reproducing device disposed to radiate sound waves approximatelyvertically into said cavity, and a rigid conical de-y fiector having avertical 'angle of the order of and disposed with its axis lying'in avertical intersecting said vertex, said deflector extendingsubstantially to the periphery of the bottom of said cavity and beingefiective to direct and distribute outwardly from said vertex the soundwaves radiated by said device.

7. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shapedto conform substantially to the vertex ofa trihedral angle, as a. corner of a room, a largediaphragm typesound-reproducing device disposed to radiate sound waves approximatelyvertically into said cavity,- and a rigid conical defiector havinga'vertical angle of from 110 to and disposed with its axis lying in avertical' intersecting said vertex, said deflectorbeing efl'ective todirect and distribute outwardly from said. vertex the sound wavesradiated by said device.

8. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner 01' a room, a largeinto said cavity,"the vertical depth of said cavity diaphragm type sound-reproducingdevice disposed to radiate sound waves approximately vertically intosaid cavity, a conical deflector positioned below said device andeffective to direct outwardly from said vertex sound waves radiated bysaid device, and additional means for effecting uniform angulardistribution of said deflected waves.

9. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavityshaped to conform substantially to the vertex ofa trihedral angle, as a comer of a room, a large-diaphragm typesound-reproducing device disposed to radiate sound waves approximatelyvertically into said cavity, a conical deflector positioned be-' lowsaid device and effective to direct outwardly from said vertex soundwaves radiated by said device, and a plurality of fins radiating fromthe interior of said cavity effecting-a uniform angular distribution ofsaid-deflected waves.

10. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a largediaphragm typesound-reproducing device dis-.

mental deflector and said fins'being effective to secure uniform angulardistribution of said deflected waves.

y 11. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a large- Y diaphragm typesound-reproducing device discluding a segmental conical deflectorparallel to said first-named deflector and a plurality of vertical fins,said segmental deflector and said fins radiating substantially from theaxis of said diaphragm to the periphery of said cavity.

12. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a horisontal partitiondefining the upper boundary .of said cavity, a large-diaphragm typesound-reproducing device disposed in an opening in said partition andradiating sound waves vertically into said cavity, and means fordirecting outwardly and distributing uniformly the sound waves radiatedby said device.

- 13. A high-fidelity sound-reproducing apparatus comprising a casingincluding a sound cavity shaped to conform substantially to the vertexof a trihedral angle, as a corner of a room, a horizontal partitiondefining the upper boundary of said cavity, a large-diaphragm typesound-reproducing device disposed in an opening in said 7 partition andradiating sound waves vertically being of the order of the diameter ofsaid diaphragm, and means'for directing outwardly and distributinguniformly the sound waves radiated by said device.

14. A high-fidelity sound-reproducing apparatus comprising a casingterminating in a sound cavity shaped to conform substantially to thevertex'of a trihedral angle, as a corner of a room, a partitionseparating said cavity from the rest of said casing, a large-diaphragmtype sound-reproducing device disposed in an opening in said partitionto radiate sound waves substantially, vertically into said cavity, meansfor directing the sound waves radiated by said device outwardly fromsaid vertex, and means for rendering the casing above said partitionsubstantially non resonant.

'15. A h gh-fidelity soimd-reproducing apparatus comprising a casingterminating in a sound cavity shaped to conform substantially to thevertex of a trihedral angle, as 'a corner of a room, a partitionseparating said cavity from the rest of. said casing, a large-diaphragmtype soundreproducing device disposed in an opening in said partition toradiate sound waves substantially vertically into said cavity, means fordirecting the sound waves generated by said device outwardlyfrom saidvertex, and means for rendering the casing above said partitionsubstantially non-resonant, comprising a lining of sound-ab- .sorbentmaterial and a mass of sound-absorbent material at the opposite end ofsaid casing from said cavity and formed with a surface inclined to saidpartition.

16. A high-fidelity sound-reproducing apparatus comprising a casingterminating in a sound cavity shaped to conform substantially to thevertex of a trihedral angle, as a corner of a room,

a partition separating said cavity from the rest of said casing, alarge-diaphragm type sound-reproducing device disposed in an opening insaid partition to radiate sound waves substantially vertically into saidcavity, means for directing the sound waves radiated by said deviceoutwardly from said vertex, and means for rendering the casing abovesaid partition substantially anti-resonant, comprising a lining ofsound-absorbent material and a mass of sound-absorbent material disposedto form a pyramid having as its base the end of said casing oppositesaid cavity and having its vertex at the corner of said casing includingthe vertex of said cavity.

17. A high-fidelity sound-reproducing appara.. tuscornprising a casingconstructed with sufiicient rigidity as to be substantiallyanti-resonant and terminating in a sound cavity shaped to conformsubstantially to the vertex of a trihedral angle, as a corner of a room,a horizontal partition separating said cavity from the rest of saidcasing, a lining of sound-absorbent material for the lastmentionedportion of said casing, a sound-pervious screen dividing a pyramidalchamber out of said casing having as its base the end of said casingopposite from said cavity, a mass of soundabsorbent material fillingsaid chamber, a large diaphragm type sound-reproducing device disposedin an opening in said partition and radiating sound waves substantiallyvertically into said cavity, and means for directing outwardly from saidvertex sound waves radiated by said device.

. HAROLD A.-WI-IEEI.-ER. VERNON E. WHITMAN. NELSON P. CASE. WILHAM O.SWINYARD.

